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Credit for awesome summary Attack on Titan Season 4 Wikipedia.
Zeke returns from Grisha's memories and desperately pleads with Ymir to enact the euthanasia plan, but as she walks toward the Coordinate, Eren rips free from his chains and rushes towards her.
In a flashback to 2,000 years earlier, Ymir lived in a small rural village which was invaded by Eldians led by Fritz who enslaved the town's populace. The young Ymir is blamed for letting a pig escape. While she is being hunted down, she enters an ancient tree and falls into a deep pool where a parasitic-looking entity turns her into the first Titan.
Still treating her as a slave, Fritz uses Ymir's Founding Titan to destroy Marley's armies and expand the kingdom of Eldia. During this time, Fritz has three daughters with Ymir: Maria, Rose, and Shina. One day, Ymir sacrifices herself to save Fritz from a dissenter, and Fritz orders his children to eat her body so their descendants can keep passing on the power of the Titans. Ymir's soul is transported into the Paths dimension where she spends the next 2,000 years making Titans by hand.
Back in the present, Eren reaches Ymir and offers her the freedom to choose her future for the first time in her life. Ymir decides her future lies with Eren and begins The Rumbling, collapsing the Walls and releasing the thousands of Colossal Titans encased within it. Now transformed into his Founding Titan form, Eren then uses the Paths to announce to all Subjects of Ymir his intention for the Titans to exterminate all life beyond the island.
NOTE: This episode takes place in volume 30 of the manga.